FIG. 2 shows a cross-section of a prior art two beam array semiconductor laser (hereinafter referred to as "two beam array LD") device. In FIG. 2, reference numeral 10 designates a monolithic two beam array LD chip. This two beam array LD chip 10 is bonded onto a silicon submount 11 which is mounted on a heat sink 12. Reference numeral 5 designates a photodiode (hereinafter referred to as "monitor PD") for monitoring light emitted by the two beam array LD chip 10. The heat sink 12 and the monitor photodiode 5 are bonded onto a stem 6. Reference numeral 7 designates a cap, and reference numeral 8 designates a window fixed to the cap 7.
This laser device will operate as follows.
The rear surface, i.e., monitor light emitted from the two beam array LD chip 10 is detected by the monitor photodiode 5 which is disposed on the stem 6. Those light signals are converted into an electric signal by the monitor photodiode 5, and the electric signal is sent to an APC (Auto Power Control) circuit located outside the laser device. The light outputs of the laser emitted from the front facet of the two beam array LD chip 10 are adjusted in response.
In the semiconductor laser device of such a construction, the two monitor lights emitted from the beam array LD chip 10 is received by the monitor photodiode 5. Therefore, when the two laser lights are operated at the same time, they cannot be controlled independently by the APC. Furthermore, they are apt to receive thermal and electrical interference.
FIG. 3 shows a semiconductor laser device disclosed in Japanese Published Patent Application 60-175476. In FIG. 3, reference numeral 14 designates a semiconductor substrate. Reference numeral 13 designates an electrode disposed on the semiconductor substrate 14. Reference numeral 15 designates an insulating region produced at a bottom portion of the substrate 14. The semiconductor substrate 14 is disposed on a heat sink 16. A plurality of heat sinks 16 for a plurality of respective semiconductor substrates 14 are insulated by insulator 17 from each other. That is, metal heat sink 16 and insulator 17 for electrode separation are arranged in a sandwich manner, and semiconductor chips are mounted on the respective heat sinks. In the semiconductor laser device of such a construction, it is possible to electrically and thermally separate the semiconductor chips from each other.
FIG. 4 shows a semiconductor laser array device disclosed in Japanese Published Patent Application 61-159788. In FIG. 4, reference numeral 18 designates a semiconductor laser array. Reference numeral 19 designates an n type GaAs heat sink. Reference numeral 20 designates a pn junction photodiode disposed on the heat sink 19. Reference numeral 21 designates an electrode. Reference numeral 22 designates a remainder left from etching required portions of the heat sink 19. The semiconductor laser array 18 includes a plurality of semiconductor laser elements disposed on the heat sink 19, and the pn junction photodiodes 20 are disposed just behind the respective semiconductor laser elements in array 18 on the heat sink 19. The remainder 22 is disposed between the respective photodiodes. In such a construction, laser light emitted from the rear facets of respective semiconductor laser elements are precisely monitored by the photodiodes 20. Furthermore, since GaAs region 22 of convex configuration lies between adjacent photodiodes, the respective laser beams can be independently monitored without thermal and electrical interferences.
In the semiconductor laser device disclosed in 60-175476, however, when an LD chip is used as a semiconductor chip, a plurality of LD chips are arranged on the same main surface. Therefore, when the respective laser beams are emitted at the same time and are monitored by a light receiving element (not shown), there arise problems of thermal and electrical interference. Furthermore, it is impossible to drive the respective laser lights independently with an APC.
In the semiconductor laser array device disclosed in 61-159788, since the respective laser elements are placed on the same surface, a plurality of pn junction photodiodes which monitor the light outputs have to be disposed quite close to the laser array even when a GaAs convex region 22 is disposed between the adjacent photodiodes. This results in difficulty in production. Furthermore, since the heat sink comprises GaAs, the heat radiation capacity of the heat sink is poor.